Net migration is a fraught figure though because it counts not only immigrants from the European Union and elsewhere but also British citizens coming and going. Net migration figures have risen in the past few years, in part because fewer British citizens are leaving the country.

"Immigration has been broadly stable since about 2006, bobbling up and down at just over half a million people a year. ... But while immigration has stayed broadly the same, the figures show that fewer people are emigrating."

Passengers board a bus leaving for Poland from Victoria coach station in May 2009 in London. Getty Images.

A decrease in those leaving rather than an increase in those arriving led to the 21 percent increase in net migration last year. The biggest change is that the number of British citizens leaving is 70,000 less than before the financial crisis. Based on those figures, if the Conservative government wants to meet its target, it should make it more attractive for British citizens to leave.

Like elsewhere, the immigration debate is an emotive not a rational one, and the competing studies this week did little to clear the air.

The group admits that correlation is not causation, "but when the two statistics are placed side by side, most objective people would consider it a very remarkable coincidence if there was no link at all between them," said the group's chairman, Sir Andrew Green.

However, MigrationWatch also reports that econometric studies looking at the impact of migration on employment "found no 'statistically significant' linkage between A8 immigration and the employment of UK workers." MigrationWatch says that no study has looked specifically at youth employment.

The group chooses to find the results of the econometric studies that don't support its anti-immigration position as "counter-intuitive".

Portes responded to MigrationWatch in the Independent: “As it says, between 2004 and 2011 an extra 600,000 Eastern European workers entered the UK labour force, while youth unemployment rose by 400,000. But most of that rise took place during 2008 and 2009, when the number of Eastern European workers fell.”

Further muddying the debate, the Migration Advisory Committee, set up by the Home Office, released a report contradicting the NIESR findings that immigration had no impact on employment. The committee said that for every 100 migrants from outside of the European Union, 23 fewer native Britons were employed.

However, the picture is complex. "Those migrants who have been in the UK for over five years are not associated with displacement of British-born workers," the committee reported, and migrants have helped increase salaries for well-paid positions but has depressed salaries for low-pay positions.

With recession putting pressure on British workers, the anti-immigration sentiment is mainstream and virulent, and it's having a corrosive affect on British politics. These competing studies have done more to confuse and little to clarify and calm the dangerously heated debate. As the Independent says: "the MAC findings will no doubt be grist to the mill of the vociferous anti-immigration lobby. Sadly, in these economically uncertain times, their arguments are acquiring more of an edge."